How to Finish Exterior Foundation Insulation
Options abound for covering up above-grade rigid foam insulation, including one that may surprise you
Energy gurus and building codes routinely recommend these days that foundation walls be insulated. One way of accomplishing that is by adding a layer of rigid foam insulation on the outside of the foundation.
And that's exactly what William Poole is planning to do.
Most of the rigid foam insulation will be underground and out of sight. But what do you do with that stretch of exposed insulation above grade?
"I just can't get excited about options for covering the insulation in the space between the top of grade and the bottom of the siding," Poole writes in a Q&A post. "Has anybody completed a project with this detail and come up with a material that is both practical and, for lack of a better term, 'good looking'?"
Here's a list of options
GBAGreenBuildingAdvisor.com senior editor Martin Holladay is first out of the box with a list of not just a few but 10 potential solutions.
He lists four possible generic products: pressure-treated plywood, fiber-cement panel siding, stucco, and surface-bonding cement. There also are a number of proprietary products worth looking at, Holladay says, including Insul-Guard 2 fiberglass from Diversified Composites, Ground Breaker fiberglass covering from Nudo Products, FP Ultra Lite Panels from Styro Industires , and Protecto Bond membrane, a peel-and-stick product from Protecto Wrap.
Robert Riversong has used surface-bonding cement with "excellent success." He adds extra acrylic mortar modifier to the mix to improve plasticity, adhesion and water resistance. And for a "belt-and-suspenders" approach, he runs 1/2-in. hardware cloth from the sill to a few inches below grade before troweling on the cement.
"It leaves a highly water-resistant and very tough 'stucco' coating that looks great and has excellent tensile strength and crack-resistance because of the fibers," Riversong writes.
The coating should be vapor permeable even with the acrylic additive, he adds, and can be waterproofed with UGL Drylok latex masonry sealer, available in both gray and white.
Fiber cement has some caveats
Allison Bailes writes that he's heard recommendations for fiber cement, but adds, "I always worry about that. I've seen scrap fiber-cement siding lying on the ground and delaminating after getting wet. Yes, if the water is managed correctly, it should never be wet for very long, but still, it's outside with the bottom in the dirt.
GBA Advisor Michael Chandler, a builder in North Carolina, uses Hardie backer made by James Hardie. Joints between panels are covered with fiber mesh tape embedded in surface-bonding cement, then covered with a sponge finish stucco and elastomeric paint.
"We didn't have any issues with delamination of the Hardie-backer, but we had to go in within a year and caulk all the vertical cracks and re-paint," Chandler says. "Since then we've been fine, but on other projects where we stucccoed over mechanically fastened Spider Lathe synthetic stucco lathe we've not had as much cracking."
Allison's concerns are well founded, says Holladay. Drawing on research he originally did for Energy Design Update, Holladay says only one fiber-cement manufacturer, MaxiTile of Carson, California, allows installation in contact with the ground.
James Hardie, Cemplank and CertainTeed all discouraged that application.
Mark Hinrichs adds that prefinished fiber-cement panels and boards and cement backerboard can be used in contact with the ground, but only in mild climates.
"But the manufacturers don't recommend them for colder climates where they will be subjected to freeze-thaw cycles," he adds. "They will begin to deteriorate after 100-150 freeze-thaw cycles. This from the James Hardie engineering department when I called to ask."
Although Riversong has had good luck with surface-bonding cement, he points to a way of sidestepping the protection issue altogether by using a Thermomass foundation.
"Another alternative, to avoid the problems of external foam board, is to build on a Thermomass foundation, which places the XPS midline in the concrete wall, with the inner and outer wythes connected 12 in. o.c. with fiberglass ties that have the tensile strength of ½-in. rebar," he says.
The foundation gets the same insulation benefit, but now the foam is out of harm's way.
And the solution that may surprise you?
"Try a spray on bed-liner application," writes NH Green Builder. "Some guys are set up to do it mobile, although it is fairly expensive it looks great."
"The idea to cover it with a spray-on bed-liner is really interesting," says Max York. "Has anyone else tried that? It doesn't sound great for the environment though."
Our expert's opinion
Here's how GBA technical director Peter Yost responds:
First, a confession: Steve Baczek, Mike Guertin and I missed this issue when we were doing the Key Materials tips for the GBA foundation construction details involving exterior rigid insulation for basement and crawlspace foundations. We did include a tip on the protective covering for slab foundations but it’s not nearly as helpful as the contributors to this Q&A post.
Second, this detail has always troubled me both aesthetically and functionally. It’s a little bit like wearing your pants below your drawers (here, I date myself, I realize… ). But we have to keep the finished grade at least 6 to 8 in. down from framed above-grade assemblies by code, so we end up with a strip of unfinished structure exposed (and on steeply sloped sites, sometimes a lot more than a strip). Back when I actually built stuff, we always applied a parge coat of latex-modified stucco with joints taped with fiberglass mesh. But it was a bit of handwaving at both the aesthetics and the function; it did not look that great and was not very durable.
I think the best looking option, but certainly not the least expensive, is cultured stone veneer. Its greater thickness is quite likely to mean a water-table detail at the transition to the above-grade wall. And for any exterior foam insulation more than ½ in. thick, you are likely to need special fasteners securing the insulation to the basement foundation wall, or even a brick ledge. The stone veneer turns this transition from a bit of an aesthetic cluge to an architectural feature. Beautiful, but pricey.
Methods described by Martin, Robert, and Michael all made sense to me. I agree that while a material like Hardie backerboard can work (and I like protective coverings that are essentially hard sheet goodsMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen., as more robust in protecting the insulation), fiber-cement siding materials are not a good fit.
In terms of vapor permeability: many coating manufacturers can now pretty much “dial in” this property, even for elastomeric coatings; just get the standardized test numbers from the manufacturer. But whether or not you need drying to the exterior depends a lot on drying potential to the interior. Concrete foundation walls don’t do much drying to the exterior, except above grade.
- Green Building Advisor
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